There is an ongoing need for component miniaturization in radio wave communication devices. For example, smaller and more efficient components are needed for light-weight, hand-portable cellular telephones, wireless local area networks for linking computer systems within office buildings in a readily reconfigurable fashion, wristwatch- and credit-card-sized paging apparatus and other devices for promoting rapid, efficient and flexible voice and data communication.
Filters are needed for a variety of such communications applications wherein small size, light weight and high performance are simultaneously required. Increasing numbers of products seek to employ fixed spectral resources, often to achieve tasks not previously envisioned. Examples include cellular telephones, inter- and intra-facility computer-computer and/or computer-ancillary equipment linkages as well as a host of other, increasingly complex inter-personal and/or equipment information sharing requirements. The desire to render increasingly complicated communications nodes portable and even hand-held and/or portable and/or pocket-sized places extreme demands on filtering technology in the context of increasingly crowded radio frequency resources.
Acoustic wave devices provide filters meeting stringent performance requirements, which filters are (i) extremely robust, (ii) readily mass produced, (iii) adjustment-free over the life of the unit and (iv) which sharply increase the performance-to-size ratio achievable in the frequency range extending from a few tens of megahertz to about several gigahertz. However, need for low passband insertion loss simultaneously coupled with demand for high shape factor and high stopband attenuation pose filter design and performance requirements not easily met by a single acoustic wave filter alone.
One approach to satisfying these needs and demands is to cascade two or more acoustic wave filters. This approach realizes increased stopband signal rejection but requires additional matching components (e.g., inductors and/or capacitors) and also multiplies the volume and weight of the acoustic wave filters by the number of such filters cascaded, when each filter is separately realized, impedance matched and packaged. Matching components additionally incur major size and weight penalties because each transducer generally requires at least two matching components, each of which is at least as large as an acoustic wave filter die.
Another approach is to provide two or more such filters on a single substrate, wherein the filters are designed to have purely real impedances matched one to another without requiring intervening matching components. One realization includes a series-parallel arrangement of resonant elements having staggered center frequencies and arranged in a ladder structure, i.e., a structure known as a "ladder filter" and comprising cascaded sections, each including a series resonant element followed by a shunt resonant element. Typically, within each section, the antiresonant frequency of the shunt element is chosen to be equal to the resonant frequency of the accompanying series element. Disadvantages of this approach when implemented employing SAW resonators include a fixed bandwidth for the electromechanical coupling coefficient (k.sup.2) associated with the chosen substrate material. Generally, conventional design approaches are such that when three of the filter material, impedance, selectivity and bandwidth characteristics are specified, the fourth is also determined.
Acoustic wave filters including ladder filters formed from groupings of resonators employ generally periodic arrays of electrodes configured to provide discrete elements such as transducers (for converting electrical to mechanical energy and vice versa), reflectors (for reversing the direction of propagation of an acoustic wave) and gaps for separating transducers and reflectors. These elements are grouped in a generally in-line configuration (e.g., reflector, gap, transducer, gap, reflector) along a principal axis of acoustic wave propagation on a suitable substrate material, with the entire array providing an electrical filtering function associated with the electrical port(s) of the individual transducer(s) and/or the composite filter.
Typically, acoustic reflectors provide reduced insertion loss by trapping energy within a Fabry-Perot-like cavity formed about a transducer. Conventional acoustic reflectors include large numbers of reflection elements in order to increase the efficiency of the energy-trapping. This increases the physical size ("footprint") of the device and also reduces the bandwidth over which the reflector operates efficiently. Additionally this may result in increased ripple in the filter "skirt" regions, where the response is rapidly changing from low to high insertion loss (or vice versa) with frequency.
What is needed is a ladder filter configuration/design methodology providing flexible bandwidth, suitable out-of-band rejection and low in-band insertion loss, drift-free performance and realizable in compact, robust and desirably in monolithic form.